Answer:
1. The First Major Mass Extinction: The Ordovician Mass Extinction.
The first known major mass extinction event occurred during the Ordovician Period of the Paleozoic Era on the Geologic Time Scale. At this time in the history of Earth, life was in its early stages. The first known life forms appeared about 3.6 billion years ago, but by the Ordovician Period, larger aquatic life forms had come into existence. There were also even some land species at this time.
The cause of this mass extinction event is thought to be the shift in the continents and drastic climate change. It happened in two different waves. The first wave was an ice age that encompassed the entire Earth. Sea levels lowered and many land species could not adapt fast enough to survive the harsh, cold climates. The second wave was when the ice age finally ended. The episode ended so suddenly that the ocean levels rose too quickly to hold enough oxygen to maintain the species that had survived the first wave. Again, species were too slow to adapt before extinction took them out completely. It was then up to the few surviving aquatic autotrophs to increase the oxygen levels so new species could evolve.
2. The Second Major Mass Extinction: The Devonian Mass Extinction
The first wave, which dealt a major blow to aquatic life, may have actually been caused by the quick colonization of land—many aquatic plants adapted to live on land, leaving fewer autotrophs to create oxygen for all of the sea life. This led to mass death in the oceans.
3. The Third Major Mass Extinction: The Permian Mass Extinction
It is still much of a mystery what set off this greatest of the mass extinction events, and several hypotheses have been thrown around by scientists who study this time span of the Geologic Time Scale. Some believe there may have been a chain of events that led to so many species disappearing; this could have been massive volcanic activity paired with asteroid impacts that sent deadly methane and basalt into the air and across the surface of the Earth. These could have caused a decrease in oxygen that suffocated life and brought about a quick change in the climate. Newer research points to a microbe from the Archaea domain that flourishes when methane is high. These extremophiles may have “taken over” and choked out life in the oceans as well.
4. The Fourth Major Mass Extinction: The Triassic-Jurassic Mass Extinction
The fourth major mass extinction was actually a combination of many, smaller extinction events that happened over the last 18 million years of the Triassic Period during the Mesozoic Era. Over this long time span, about half of all known species on Earth at the time perished. The causes of these individual small extinctions can, for the most part, be attributed to volcanic activity with basalt flooding. The gases spewed into the atmosphere from the volcanoes also created climate change issues that changed sea levels and possibly even pH levels in the oceans.
5. The Fifth Major Mass Extinction: The K-T Mass Extinction
The Cretaceous-Tertiary Mass Extinction (or K-T Extinction) became the dividing line between the final period of the Mesozoic Era—the Cretaceous Period—and the Tertiary Period of the Cenozoic Era. It is also the event that wiped out the dinosaurs. The dinosaurs were not the only species to go extinct, however—up to 75% of all known living species died during this mass extinction event.